The pulmonary endothelial enzyme Angiotensin Converting Enzyme (Kininase II) has been proposed as a marker suitable for detection of pulmonary endothelial injury and repair. During acute lung injury, ACE can be found in increased amounts in alveolar lavage fluid, and, to a lesser degree, in plasma. However, no information is available about the biosynthesis and turnover of ACE in the intact lung, information needed for interpretation of studies of ACE as a marker. The rate of biosynthesis of ACE will be determined using radiotracer techniques in awake rats. Tritiated leucine will be infused intravenously for brief periods and the rate of incorporation of 3H-leucine into immunoprecipitated, electrophoretically purified ACE determined. Simultaneous measurement of the specific radioactivity of leucyl-tRNA of lungs from the same animal using recently developed micromethods will provide knowledge of the protein synthesis precursor which are needed for determining the absolute rate of ACE biosynthesis and turnover. This approach, when applied to models of acute lung injury induced by intratracheal bleomycin, hyperoxia, and alpha naphthyl thiourea will for the first time allow determination of how the amount of ACE in the lung is modulated by biosynthesis and turnover. Similar measurements in lavage fluid will indicate the source of the ACE 2 which is shed into the alveolar spaces. These studies will provide a rational basis for use of ACE as a marker of pulmonary endothelial injury occurring during ARDS in man.